This is in my 67 chevelle. I am throwing this system together with a new HU and 6x9s and everything else was laying around my garage so it has bee a budget DIY system.

I hooked up a pair of 3 way 100W rated (400peak) 6x9s (GTX6931 Sony) to 1 channel each of my 4 channel amp. (RockFosgate 160.4) (front channel set to full range)
I hooked up my 12" Alpine sub to one channel on the other side, with one channel left untouched for the rear. ( set to low pass)

The sub sounds great like it used to when the ampwas dedicated just to the sub.

The 6x9s though are not very loud. They distort at higher input volume but they aren't producing much sound. I will have the HU cranked to max with the gain at about max and it is still quieter than the 6" in my boat that run just off the HU. The amp states that it is 20w x4 at 4 ohm and 40x4 at 2 ohm.

My head unit is not connected to any speakers, just 2 pair of RCAs to the amp.

I am fine with the subs performance. How can I get more volume from the 3 way 6x9s?

My uneducated ideas are as follows:

1) Run the 6x9s in series off one channel like I did my tower in my boat which sounds great but thats from a much larger amp.
Then do I need to bridge my sub so the amp is essentially bridged on both sides??? The amp is labeled for bridging on both front and rear channels

2) Other idea is to run the 6x9s off the HU (50x4) and leave the sub and amp alone.

3) Buy another amp to run the 6x9s

Eventually I want to get some 5 1/4s for under the dash I just need to work out mounting, I don't really want them in the doors or kick panels as this is a 67 chevelle and I want the interior looking completely stock aside from the 6x9s in the package tray. I had a mustang back in the day where the tape player, yeah thats right, just ran two 6x9s in their own boxes and it was much louder than this system has turned out.

I am guessing I am only getting about 20w to each 6x9 and that is causing low volume and distortion??